When a vehicle such as an automobile is started or accelerated, strong driving force transmitted from an internal combustion engine (hereinafter, referred to as an engine) or a motor (hereinafter, referred to as an electric motor), which is a driving source, may cause tires to spin (wheelspin) from time to time, due to the relationship between the friction coefficient of a traveling road surface and the driving force. In many cased, the wheelspin not only wastefully dissipates the driving force but also makes travel of the vehicle unstable, and hence poses a big problem also in terms of safety.
In general, in traction control of a vehicle, a wheelspin is comprehended based on the vehicle speed, the respective rotation speeds of the tires, and the like, and then driving force from the engine or the electric motor is adjusted so as to be reduced, so that the wheelspin is eliminated. As a result, especially on the road surface whose friction coefficient is decreased due to a snowfall or the like, the safety of the vehicle can be raised independent of the ability of a driver. However, in a conventional vehicle whose driving source is an engine or in a hybrid vehicle whose driving sources are an engine and an electric motor, the diver is required to finely adjust stepping-on of the accelerator pedal so as to prevent a wheelspin.
In contrast, the torque response of an electric motor mounted in an electric automobile is more than 10 times as fast as that of an engine; thus, even software alone may realize traction control whose performance is superior to that of traction control in a normal engine automobile.
In a conventional traction control apparatus for an electric automobile, disclosed in Patent Document 1, the occurrence of a slip of the driving wheel is determined based on whether or not the rotation speed of the driving wheel detected by an encoder-type wheel speed sensor has exceeded a threshold value, and then the driving torque is suppressed. However, in the case of this conventional apparatus, no slip of the driving wheel can be determined in a low-vehicle-speed region where the encoder-type wheel speed sensor cannot detect a slip; therefore, an initial slip at a time of starting cannot be suppressed.
Accordingly, in a traction control apparatus disclosed in Patent Document 2, for which an application for a patent has been made by the applicant, a virtual speed of a driven wheel is created in a low-vehicle-speed region where an encoder-type wheel speed sensor cannot detect a slip, and then the output torque is controlled for a target slip rate by use of the created virtual speed. In other words, the slipperiness of a road surface is determined based on an accelerator-on time and the rotation speed of a driving wheel (referred to also as a driven wheel; however, referred to as a driven wheel, hereinafter), and then the virtual speeds of the driven wheel are switched in accordance with the determined slipperiness of the road surface. The foregoing virtual speed is calculated in such a way as to become a smaller numerical value as the road surface is more slippery.